1. Field of the Invention
The present invention relates to a method of manufacturing a metal/polymer composite article. More particularly, the present invention relates to a method of forming a metal/polymer composite article by spraying molten metal and polymeric materials to form articles composed of metal and polymer admixtures.
2. Description of the Related Art
There are several motivations to produce material article that incorporates both metallic and polymeric phases. The metal provides strength and durability while the polymeric material reduces the weight of the article and provides for lower frictional properties or allows for chemical interaction to occur through the article. While many possible applications exist for metal/polymeric composite materials, their manufacture has been difficult and expensive. Generally, the temperatures needed to melt metals of technological interest will vaporize most polymers.
Materials that have improved wear resistance, self lubricating, and or thermal insulating properties have been prepared by thermal spray processes. These materials have generally been applied atop a metal article as a thin coating. For example, U.S. Pat. No. 5,837,048, teaches a plasma spray coating of polymeric cellulose ether with a metal or ceramic powder. Between 1 and 10% by weight of the polymeric material is combined with the metal or ceramic and applied as a plasma spray feedstock. The polymeric, metal and ceramic materials are blended together combined and sprayed using a spray gun. The invention describes the complexities of spraying the mixture through a single spray gun. The spray temperatures for spraying metal and polymeric materials are different and the metal and polymeric materials tend to separate.
U.S. Pat. Nos. 5,434,210, 5,766,690 and 5,464,486 also teach methods of combining friction-reducing materials with metals and ceramics to produce powders that can be formed into abradable seals using thermal spray. Again, the metal and friction reducing material are premixed and applied using a single thermal spray gun. The mixture forms a relatively thin coating that is applied to a metal article. However, thermally spraying premixed metal/polymer or ceramic/polymer powders often produce unacceptable end results because the optimal conditions required (temperatures, type of projecting gas, voltage, current) metals, ceramics and polymers are significantly different. Consequently, the thermal spray parameters that optimize the microstructures and properties of one phase often produce undesirable chemistry and properties of the other.
Another use of a metal/polymeric article is as a separator for an electrical or chemical article. U.S. Pat. No. 5,021,259, teaches a method of applying a thermoplastic coating onto a porous metal surface by thermally spraying the thermoplastic polymer. The porous metal and coating are then heated to fuse the thermoplastic polymer coating into the porous metal. The metal supports the polymer and forms a protective covering for the metal. This patent additionally teaches a method of infiltrating a polymeric material into the surface of a metallic substrate. The polymer is applied as relatively thin coating atop a metal substrate. The metal substrate must first be formed to have the desired porosity network. The polymer coating must be melted to cause the coating to flow into the pores. Because of the relatively low viscosity of polymeric materials, the polymer only penetrates the area nearest to the surface of solid metals.
A relatively new material combines polymeric and metal materials into a single particle that can he used as a thermal spray powder feedstock. U.S. Pat. No. 5,660,934 teaches methods for manufacturing clad plastic powder particles suitable for thermal spray. These powder particles, consisting of a plastic core surrounded by ceramic or metal particles, can be thermal sprayed because the outer ceramic and metal particles protect the inner polymeric material for the high thermal spray temperature. These onerous ceramic or metal encapsulated polymeric particles are often used as a small fraction of an overall thermal spray feedstock material.
The salient feature of all of the above is that they teach various methodologies of improving the surface wear and corrosion properties of metallic articles using metal/polymer or ceramic/polymer composite coatings. In all cases the metallic substrate provides the bulk properties while the coating provides desired surface characteristics. These articles always require dual bulk and surface manufacturing steps and their useful life usually terminates once the surface coatings are removed. The cited references do not teach any methodology of making a complete article that incorporates intimate mixtures of metal and polymeric materials in its bulk. Additionally they do not teach the use of co-deposition techniques, using multiple and different thermal spray guns to form solid articles containing polymeric and metallic admixtures.
Traditional valve seats for sealing around poppet valves in internal combustion engines maybe made of sintered powdered metal compacts or alloy castings. Casting and sintering processes often require temperatures in excess of 1000.degree. C. and limit the compositions available for use as valve seat inserts. Desirable solid lubricating materials such as MoS.sub.2 and BN cannot be easily incorporated into the valve seat material because they either decompose, sublime, or fail to provide wetting at the melting or sintering temperatures of most metals. Traditional valve seats have not incorporated polymeric material because the processing temperatures needed to incorporate the polymeric material into the valve seats exceed the decomposition, boiling or degradation point of most polymeric materials.
The need for self lubricating valve seats is extremely important for compressed or liquid pressurized natural gas (CNG or LPG) fueled engines. Gasoline fuels contain additives that provide some degree of lubrication to the valves; especially the intake valves. Natural gas does not provide any lubrication to the valves. They run virtually dry. Consequently, traditional valve seats do not provide the required engine durability. Harder valve seat inserts particularly those containing significant amount of cobalt, molybdenum, chromium and lead have been used with natural gas engines but these components are much more costly than traditional valve seats inserts. Liquid sodium filled ultra light valves have also been used to reduce the heat buildup and the spring load between the valve and valve seat. These products are also expensive and can be problematic in case of unanticipated valve failure.
The present invention overcomes all of the above limitations and enables the manufacturing of a low cost metal/polymeric article that has polymeric material throughout the bulk thus providing the article with better friction and wear properties and extended life. The present invention also produces an article in a single step without the need for separate bulk and surface processing. The process incorporates simultaneous metal and polymer processing methodology to form metal/polymer composite article having required bulk and surface properties.